It is known that when a steel strip is used for producing a shaped article, for example, the body of an automobile, the steel strip is pressed into a desired shape, and the shaped steel strip is subjected to a phosphate treatment in which a phosphate film is formed on the surface of the shaped steel strip and then the phosphate-treated steel strip is coated with paint. In this case, the steel strip needs to exhibit a capability of forming a phosphate film firmly bonded onto the surface thereof and a capability of firmly bonding the paint coating having an excellent corrosion-resistance on the surface of the phosphate coating.
The capability of forming a phosphate film on a surface of a steel strip when the steel strip is subjected to a phosphate treatment is referred to hereinafter as the phosphatizing property of the steel strip.
The capability of firmly fixing a paint coating resistant to corrosion to the surface of a steel strip when the steel strip is coated with paint is referred to hereinafter as the corrosion-resistant paint coating-bonding property of the steel strip. It is also known that it is possible to produce a steel strip having a satisfactory phosphatizing property and a satisfactory corrosion-resistant paint coating-bonding property by a conventional finishing process comprising the steps of surface-cleaning, for example, by an electrolytic degreasing procedure; heating the surface-cleaned steel strip which has been wound to form a coil to a recrystallizing temperature thereof or higher in a reducing atmosphere in a batch type box-shaped annealing furnace; uniformly heating the steel strip at the above-mentioned temperature in the reducing atmosphere for a predetermined time; first cooling, in the reducing atmosphere, the uniformly heated steel strip to a temperature at which the surface of the steel strip is not oxidized; removing the thus cooled steel strip from the annealing furnace; further cooling the removed steel strip to a temperature at which no aging occurs in the steel strip and, finally, temper rolling the further cooled steel strip.
The above-described conventional finishing process includes a number of steps and, therefore, is complicated and sometimes too troublesome when it is necessary to connect the steps to each other. Also, the initial heating, uniform heating and first cooling procedures are successively applied to the coiled steel strip in the box-shaped annealing furnace. These procedures cause the finishing process to be prolonged. Therefore the productivity and economic efficiency of the conventional finishing process are unsatisfactory.
In order to improve the productivity and economic efficiency of the finishing process, various attempts have been made to simplify and/or continuously carry out the finishing process.
In order to heat the steel strip to the desired annealing temperature with a high thermal efficiency within a short period of time, an attempt has been made to directly heat the steel strip in a continuous annealing furnace. This direct heating method is effective for utilizing the heat generated in the continuous annealing furnace with a high efficiency and for increasing the heating rate for the steel strip so as to shorten the heating time for the steel strip.
In order to rapidly cool the annealed steel strip to a desired temperature within a short period of time, an attempt has been made to utilize jets of water or a mixture of water and a gas, for example, AX gas which is a mixture of nitrogen and hydrogen. This jet cooling method is effective for allowing the cooling rate of the steel strip to be easily changed over a wide range in response to the desired quality of the steel strip and for permitting the cooling procedure per se to be readily stopped when the steel strip reaches a desired temperature.
However, it has been found that when the steel strip is subjected to a direct heating type annealing procedure, the surface of the steel strip is oxidized while the temperature of the steel strip is being elevated even if the air ratio is strictly controlled. The oxidized surface is reduced in the next stage of holding the steel strip at the elevated annealing temperature in a reducing atmosphere. In this reduction, since the surface of the steel strip comes into direct contact with the reducing atmosphere, the surface is excessively activated. The activated surface of the steel strip exhibits a poor phosphatizing property.
That is, when the surface of the steel strip is subjected to bonderizing procedures, the crystals in the resultant phosphate film are coarse.
Also, when the steel strip is cooled from the annealing temperature to an overaging temperature or lower by jetting water or a mixture of water and a gas, for example, the AX gas, the surface of the steel strip is oxidized, sometimes to the extent that the average thickness of the resultant oxide layer reaches 500 angstroms or more, for example, 2000 angstroms. The overaging procedure is carried out at a relatively low temperature of 200.degree. to 500.degree. C. in a reducing atmosphere. Therefore, it is impossible to satisfactorily reduce the oxide layer on the steel strip in the overaging procedure. Accordingly, on the overaged steel strip the remaining oxide layer has an unsatisfactory appearance and exhibits a poor corrosion-resistant paint coating-bonding property.
Proposals have been made to eliminate the disadvantages of the above-described annealing-cooling method. Japanese Patent Application Laid-open No. 53-132418 discloses a process for removing the oxide layer from the overaged steel strip by introducing the overaged steel strip into a pickling apparatus located downstream of the overaging furnace.
However, it has been found by the inventors of the present invention that the removal of the oxide layer by the pickling procedure causes the surface structure of the resultant pickled steel strip to be excessively uniform. This excessively uniform surface structure causes the surface of the steel strip to exhibit a poor bonderizing activity. Therefore, when the pickled steel strip is subjected to a phosphatizing procedure, the nucleuses of the bonderizing crystals are formed only in a restricted area and the bonderizing crystals are coarse. This phenomenon results in a poor corrosion-resistant coating-bonding property of the steel strip.
It has been found that the above-described disadvantages resulting from the pickling procedure applied to the steel strip after the overaging procedure can be eliminated by descaling the surface of the steel strip and by forming a defective metal deposit layer incompletely covering the descaled surface and consisting of at least one member selected from the group consisting of nickel, manganese, cobalt, copper and molybdenum.
It is strongly desirable that the descaling procedure and the procedure for forming the defective metal deposit layer be integrated with the annealing procedure so as to make it possible to carry out the overall process in a single process line and continuously at a high efficiency.